BeforeTale
by R Kerns
Summary: Many people have their own head canons to before the events of Undertale. This is my version. join Sans, Papyrus, and Gaster as we go from the creation of the DT Extractor to the problems with the CORE. Each chapter is one year later. Undertale is owned by Toby Fox. (This is meant to be a prequel to another series I will write after this)
1. Chapter 1- Blueprints

Long ago, way before the events of Undertale, Sans was heading home after an amazing field trip to the castle in New Home. All that went through his head was how he got to meet the king himself. Asgore was incredibly nice and instantly recognized Sans. "You must be Gaster's son. I've heard so much about you." He said to Sans and all of his classmates stared in awe, amazed to know that the king actually knew him. It was definitely one of the best days of his life.

He reached home and immediately went to his father, "Dad, Dad, Dad! Today was the best day of school EVER!" The older skeleton giggled at the hyper-activeness of his son. "Quiet down, Sans. Your brother is sleeping."

"Oh, sorry"

"It's okay, Sans"

The young skeleton calmed down and told Gaster about his day at school. After Sans finished, he asked if he could visit again. "Well, maybe next time I go to see Asgore, I _might_ bring you along." At the sound of that, Sans began to jump around excitedly but stopped when he remembered that Papyrus was still sleeping.

But it was too late when both skeletons heard a distant noise from the other room. Gaster went immediately to get Papyrus. When he brought the small baby bones in to the living room, a smile covered most of Sans' face.

"Hey little bro"

"Hai Snans!"

Sans smiled at the sound of the voice of his little brother. It was adorable. He hoped Pap would always stay this small. He was probably hungry. Sans was also. Gaster cooked some spaghetti for the three of them.

After dinner, Papyrus was sent to bed. Sans was also getting ready to sleep when Gaster stopped his son. "Wait. I want to show you something." Gaster took Sans to his lab and he was in awe by all of the crazy gadgets and trinkets laying around. Sans was so amazed that he didn't even realize he walked straight into a wall. "What is all of this" Sans asked.

"Well, if I were to talk about every invention here, we would be here for 5 years"

"Okay. Then what about this one?"

Sans pointed to a separate room with a window. Gaster explained what it was.

"That is one of my newest projects. It is to study the science of timelines."

"What are timelines"

Gaster explained the basics of timelines to Sans and he seemed to understand it.

"Even with everything I told you, this is relatively new research. All this information is just an educated guess.''

Sans was very interested in this and he really wants to help.

"Can I help with this?"

"Well… I guess you can watch b- "

With that Sans was excited and grabbed a lab coat. Gaster was happy to see him so enthusiastic but hoped Sans would get bored. His work is very dangerous and Gaster would never forgive himself if Sans got hurt. Afterall, Sans is still very young, only seven. He knew it would only be a phase and Sans would grow out of it.

Sans was able to find a lab coat that was just his size and quickly put it on.

"So, what will we do first Dr. Dad" Sans said. Gaster grinned at the sound of his nickname.

" I think you should go to bed first. You have school tomorrow" Sans reluctantly went to his room and fell asleep.

Gaster spent the rest of the night doing more research. He noticed that the timelines can be overridden by a power he discovered, called 'determination'. Only humans had this power, but what if there was a way for monsters to harvest that power and possibly 'reset' to before they were trapped underground. Gaster suddenly came up with an idea. He wrote some blueprints and then left to go to bed.

The next day, when Sans got home, he immediately did his homework then went to the lab. He noticed that Gaster was building something new.

"What are you building?" Sans asked

"This is the 'Determination Extractor', 'DT Extractor' for short. It is made to take the determination from this"

Gaster held a red heart. Sans knew it was a human soul.

"How did you get that?"

"I told the king that by extracting the determination, I could put it into monsters and I would be able to harvest their souls when they turn to dust." Sans remembered from school that a monster soul dissipates when they die.

"Can I help build it?" Sans asked

"I guess. It's not dangerous right now"

Sans smiled and grabbed some of the materials. The skeletons worked until they finally finished building the exterior.

"Sans, this is very dangerous so I would advise that you wouldn't help here. Maybe you can play with Papyrus." Gaster told Sans. Sans agreed and went to his and papyrus' room.

"Sup bro" Sans told his two-year old brother.

"Hai!"

"What you doing?"

"Drawling"

"Really? Can I draw something too?"

"Yah!"

Sans and Papyrus drew pictures. Papyrus drew scribbles on the paper while Sans drew himself, Pap, and Gaster. Happy with what he drew, Sans put it into the pocket of his light blue jacket. When Papyrus finished his drawing, Sans took it to the kitchen and put it on the refrigerator.

Sans thought Papyrus must have been hungry, so he cooked some tacos, the only thing he knew how to cook, and ate dinner. He made extras incase Gaster would finish his invention.

After Papyrus went to bed, Sans went to the lab to check on his dad. When he entered, he saw that Gaster was asleep. Sans put a plate of tacos beside him and left for his room to sleep.


	2. Chapter 2- Blue

One Year Later…

Sans had to be on his best behavior today. The king was coming to their house to check on their inventions. Sans was told not to enter the lab so he wouldn't break something and get hurt. However, he could not ignore the constant sounds. He snuck down to the lab and watched what they were doing.

"You see, king. This machine here lets us look at the timeline as a whole and see what has, is, and will happen in this timeline."

"I see. I would love to see how it works."

"Ah, yes. You see, the radiation caused from the machine could be very dangerous, even fatal, to monsters. Which is why I have it in a separate room."

Gaster pressed a button and the machine began to glow a light blue hue. Asgore was amazed and couldn't wait to see it work.

"This isn't finished, so it might be unable to work."

Shouldn't have said anything. A loud buzzer went off signaling that something went wrong. Gaster tried his best to fix it but to no avail. The blue glow shined brighter.

Sans knew the only other way was too unplug it. Only problem was, it was inside with the machine. Being as quick as he could, Sans ran to the machine and unplugged it. Gaster yelled for his son to get out of there, but Sans didn't hear him. As the machine turned off, a large explosion engulfed the room, along with sans. He could feel it hit most of his left side. When the smoke faded, Sans could barely stay standing. His left eye felt like it was on fire. He could hear his name being called but it sounded like an echo. The last thing he saw was blue before he passed out.

When Sans woke up, he noticed he was on the couch. He groaned. Gaster came right to his side.

"Oh, thank goodness, you're finally awake. I was so worried. When I saw you were left with 1hp, I thought you might have gone into a coma. You might want this."

Gaster handed Sans some spaghetti to bring his health back up.

"thanks, how long was i out?" Sans asked. He was too weak to capitalize what he was saying.

"About an hour, or two. Asgore left when I made sure you weren't going to turn to dust. Speaking of which, WHAT WERE YOU THINKING?"

"sorry, dad. the only way was to unplug it."

Gaster sighed "well, your _heart_ was in the right place."

After Sans finished his spaghetti, Gaster checked his health once again. Gaster's face was both shocked and confused. Sans was still at 1hp, instead of 50.

"What's wrong, Dad." Sans asked, now able to capitalize his letters again.

"Are you… feeling okay?"

"Yeah…"

"You're still at 1hp."

"WHAT!" Sans was scared. Was this permanent? What caused this to happen? Would he die if someone high-fived him?! Gaster could see him shaking from fear. He WILL fix this. Even if he dies in the process.

"Can you tell me what happened from your point of view?" Sans explained what he saw, but Gaster was still confused. He realized it was 9:00pm.

"Do you want me to carry you to your bed?"

"It's okay, I don't mind sleeping here for tonight." Gaster gave him a blanket and a pillow and tucked him in.

"Don't worry, Sans. We will fix this." He kissed Sans' forehead "Love you." Then he went to his room to sleep.

When Sans woke up from his sleep, he was in his room. Weird?

"Sans?" He heard his little brother call his name "I didn't rembor you coming up here last night"

"Me nether, bro. Hey, want to hear a joke?" The three-year-old nodded. "What do you call a fake noodle? An Inpasta." Papyrus giggled at the joke. He always found Sans' puns funny.

Sans wondered if he was being called in sick for 2nd grade. It didn't matter anyway. It was the last week of school. He went through his day normally and sometimes forgot that yesterday's incident even happened.


	3. Chapter 3- Starting the CORE

**AN: Hey guys! I just wanted to clear some misconceptions of my headcanon**

 **Sans is older than Papyrus by 5 years**

 **All of the 'main' characters will show up as time goes on.**

 **As of this time in the story, only 7 humans have fell (including Chara)**

 **If anyone asks anything else about this story, I'll answer it in an author's notes**

 **Now with that out of the way, back to the story.**

ONE YEAR LATER…

"Sans, Papyrus, come quick" Gaster shouted. Both skeletons came running into the living room.

"What is it, Dad?" Sans asked, worried that something bad happened.

"I finished the blueprints for the CORE"

"Awesome"

"I know. We're starting construction first thing tomorrow" Tomorrow? Tomorrow is Saturday. What will Sans and Papyrus do?

"Don't worry. One of my assistants will watch over you two so things won't turn out like _last time."_

"Hey! How was I supposed to know what temperature to boil spaghetti at?" Sans protested. Gaster was never going to let that go, was he?

The next day, Gaster left to Hotland before Sans and Papyrus even woke up. The brothers were greeted to the sight of a small, yellow dinosaur in a lab coat.

"H-hi! I'm Alphys." Sans and Papyrus were about to introduce themselves when Alphys suddenly stopped them.

"N-no! Let me try to guess." She looked at Papyrus first.

"You must be Sans." Sans was trying hard to hold back his laughter. Alphys blushed a little, realizing her mistake.

"J-just k-kidding. I know you're Papyrus" Her face was as red as a tomato.

Sure. Sans thought.

"Why aren't you helping Dad build the core?" Papyrus asked.

"W-well, I'm s-sort of freaked out by Hotland. The fact that I would be standing r-right next to lava frightens me." Alphys shivered. Sans understood.

Ever since 'the incident', Sans has been very cautious there, making sure he wouldn't get burned. That only happened once, but Sans was able to think of a witty comeback.

"It's okay. I'm scared of a lot of things, but I have a trick to stop it." Papyrus responded

"Whenever I'm scared, I tell myself that I am great"

"He sometimes calls himself 'The Great Papyrus'" Sans interrupted.

Alphys smiled. "That's a really good idea."

"Pretty smart for a four-year-old." Sans said with pride

After spending hours talking, eating instant noodles, and watching anime, Alphys sent the two skeletons to bed. Sans was most reluctant, but still obeyed. Alphys spent the rest of her night watching Mew Mew Kissy Cutie. Half way though rewatching season 1, Alphys noticed a faint blue glow come from the kitchen. She was surprised to see Sans looking though the fridge for some milk. Not only that, but his left eye socket glowed blue.

"S-Sans!" The skeleton didn't notice that she was there. "W-why are you up?"

"I, um, couldn't sleep."

"And how are you doing _that?"_ Alphys gestured to Sans' glowing blue eye.

"This? I could always do this. Ever since I-I" Sans couldn't bring himself to talk about what happened last year.

"Does your dad know that you can do this?" Sans shook his head. He knew Gaster would freak out if he told him.

"Hmm. Well, it's still late. You need rest."

"Before I go, I want to show you something." Sans used his magic to levitate a glass of milk which surprised Alphys at first. Then, with a snap of his fingers, he seemed to have disappeared into thin air. Alphys knew he must have teleported to his room.

At around 12, Gaster finally came back from Hotland.

"W-welcome back, Dr. Gaster"

"Hello, Alphys. I hope the boys weren't much of a problem."

"N-no, not at all. Papyrus was very sweet, and Sans was very polite."

"There were no problems, I guess?"

"W-well…" Alphys paused for a moment. Should she tell him about Sans' abilities?

"N-no. Not at all."

"Great." As Alphys started to head out, she remembered something.

"H-how long will construction last?"

"Probably a few years but I won't need to be there every day. If I need you to watch the boys again, I will ask you."

"O-okay. See you tomorrow."

During the night, Sans kept getting weird dreams where he would wake up in a cold sweat. His dreams consisted of him being in the judgement hall in the castle and… and… it was blurry, but Sans knew it was important. He's been having the same dream every night for a year. Each time, he would remember something more. This time, he could hear an older man's voice saying "do you want anything?" then the sound of dust being scattered.

Sans didn't know why he had these dreams. Was it in the future? Were his dreams warning him of a threat? Was that voice Sans when he's older? He didn't know. For now, he will treat the dreams as random nightmares. He won't tell anyone, especially Gaster. He would freak out.

Sans went back to sleep and tried to endure whatever the nightmare brought next.

This time, the dream was in Snowdin. He could clearly see an older skeleton in a dark-blue hoodie and black shorts. It was him, but older? Older Sans stopped at a pile of… Was that dust?! He picked up a red scarf. Sans sobbed, "i-i'm sorry, papyrus. i should of known b-better."

He put the scarf around his neck. "that kid **WILL PAY**!"

Sans woke up, crying. Was that the future? It couldn't of been. Sans would NEVER let that happen. He tried to wipe the tears off his face, but to no avail. He then heard his brother.

"Sans. Why are you up so late and…? Are you crying?"

"N-no, bro. I just got something in my eyes."

"What did you get?"

"Tears."

Sans got up and hugged his little brother.

"Promise me something. If you're ever in danger, call out for me. I will come."

"Brother? What's wrong? Did you have a bad dream?"

Sans cried once more. Papyrus started to tear up too.

"Don't tell Dad."

Papyrus nodded.

They went back to bed and spent the rest of the night without any problems.


	4. Chapter 4- Battles

**AN: Hey guys. Sorry it's been a while. Writer's block. Anyways, here's the next chapter.**

ONE YEAR LATER…

"Ok Papyrus. If you just focus, you can be prepared for a battle." Gaster was helping Papyrus with his battle attacks.  
"Wouldn't it be better if I practiced with someone. Like Sans?" Sans was also there, cheering his brother on.  
"Absolutely not. Sans is too fragile to battle with." That was true. With 1hp, Sans wouldn't be able to last a turn against anyone.

Papyrus was devastated. He hasn't spent much time with his brother since Sans began to watch the construction of the CORE.  
"I'm sure I'll be fine. As long as I dodge. There's nothing to worry about." Sans protested.  
Gaster sighed and finally agreed.

They started the 'battle'. The area around Sans and Papyrus began to fade into black and white. In front of both of them were 4 options that read: "FIGHT, ACT, ITEM, and MERCY" they both chose FIGHT. Sans attacked first. He decided to go easy on Papyrus. Blue bones began to rise from the ground and head for Papyrus. Papyrus knew that he shouldn't move though blue attacks. He stayed perfectly still.

It was now his turn. He copied Sans and sent blue bones his way. Sans stayed still and grinned. What they both didn't expect was for him to turn blue and fall to the ground.

"Good job, Papyrus." Gaster said. The attack took a lot of energy to do. Papyrus was really tired.  
"How about we take a break for right now." Both of the skeletons agreed and went to rest.

An hour later, training began again. This time it was Gaster against Papyrus. Before ether of them attacked, Gaster said, "Remember, try to focus on your surroundings. Use it to your advantage." Papyrus nodded and began the battle. He used both normal and blue attacks then turned Gaster blue and quickly sent a ton of bones. Gaster kept slipping and got hit a lot.

"Okay. I think that's enough training for today." Gaster said tired. Papyrus was so happy that he was getting better with his magic.  
"Soon you might even be able to use it out of battle." Sans told his brother. Papyrus smiled and went to get something to eat. Sans was about to go after him but Gaster stopped Sans.

"You did very well, Sans. Papyrus is lucky to have someone to look up to."  
"Yeah. Although if we're ever in danger, Papyrus would be the one fighting."  
"Yes, but are you in danger now?  
"No"  
"Then Papyrus will look up to you until that day comes."  
Sans nodded and went to eat.

The next day, Sans went to watch the construction of CORE. He liked seeing how things are made, but he also found lack of chaos calming. He would think about what he has done and decide whether those choices were the right ones.

This time, he was thinking about what Gaster said. Sans knew something will happen to Papyrus. He has to stop that, but can he mess with future? What if Sans can't prevent it? He knew it would take place some years in the future, so there was time to plan.

Sans decided to head home. He hoped that he could take his mind of things. Sans has more stress than a normal 10-year-old. He must make sure no one knew about it. They wouldn't understand. They would brush it off and call it a nightmare, but it wasn't. The dreams are too real to be his imagination. What could he do?


End file.
